1. Field of the Invention
The present invention pertains to electron microscope specimen positioning systems (i.e., goniometers) and more particularly to a computer controlled goniometer.
2. Description of the Related Art
In an electron microscope, a set of magnetic lenses focuses a bundle of electrons onto a specimen. Electrons leaving the specimen are focused by another set of magnetic lenses and imaged onto an electron sensitive layer of some kind, such as the surface of a fluorescent viewing screen or an electron sensitive emulsion on a film or plate to create a micrograph.
The optical image created by a fluorescent screen also may be captured by a still, video or movie camera, recorded on light sensitive film or electronically on tape, and displayed as a photograph, movie or in real-time on a display monitor or TV. While conventional cameras may be used via an observation port, special cameras are also available for improved imaging on a display monitor or TV screen. For example, the Model 694 Slow Scan Camera made by Gatan Inc., 780 Commonwealth Drive, Warrendale, Pa. detects the electron image with either a single-crystal YAG or powder phosphor scintillator and fiber-optically couples the detected image to a high resolution charge-coupled device (CCD). The CCD image is then scanned to produce signals suitable for display on a monitor. This Gatan camera provides an image on a display screen which is magnified more than 20 times larger than an image produced directly by a conventional fluorescent viewing screen.
The optical image is viewed by an operator either directly on a fluorescent viewing screen or on a display monitor and assists the operator in controlling the electron microscope. Operation generally consists of mounting a specimen in a specimen holder, placing the specimen holder in a multiple axis mechanical positioning system called a goniometer, and then changing the position and orientation of the specimen about the movement axes, all while the operator is looking at an optical version of an electron beam image or pattern produced by the specimen, until a desired image or pattern is obtained.
In a conventional electron microscope, the specimen is carried near the end of a rod, which extends through a spherical pivot in or around a port of the electron beam column, so that the rod can swing in two directions, generally horizontally and vertically. The rod is also movable in an axial (i.e., radial) direction and can be axially rotated as well to tilt the specimen. Some specimen holders also allow the specimen to be tilted in a second direction and provide mechanical linkage within the rod for control of such tilt. The rod assembly part is usually removable from the mechanical positioning system which controls it and is called the specimen holder, while the mechanical positioning system into which the specimen holder fits and which is used to control the position and orientation of the specimen held in the specimen holder in combination with the specimen holder is called a goniometer. Sometimes the mechanical positioning system alone (i.e., without the specimen holder) is called a goniometer. In the following description therefore, the word "goniometer" may at times refer to only the mechanical positioning system and at other times refer to the combination of the mechanical positioning system with the sample holder.
Adjustments in all directions of adjustment are made directly by the operator, generally by turning knobs. Normally, one knob changes the position of the specimen in the radial direction along the axis of the specimen holder (hereinafter called the goniometer axis). A second knob swings the goniometer axis about the spherical pivot point in a first angular direction and a third knob swings the goniometer axis about the pivot point in a second angular direction perpendicular to the first angular direction. Since these swing angles are typically small, the positional adjustments to the specimen made by these three knobs typically approximates an X-Y-Z cartesian coordinate system (and the operator typically thinks adjustments are being made in a cartesian frame of reference), though in reality the specimen is being positioned by knobs which move the specimen in a polar coordinate system.
The orientation of the specimen holding stage in the specimen holder is also controllable typically via one or more knobs. One knob typically rotates the specimen holder about the goniometer axis, thus tilting the specimen in one direction of tilt. If only one direction of tilt adjustment is provided, the goniometer is called a single-tilt goniometer. When tilt adjustment is provided in two directions of tilt, the goniometer is called a double-tilt goniometer. The second direction of tilt is provided typically by supporting the specimen holding stage at the end of the specimen holder rod so as to allow it to be rotated about an axis that is perpendicular to the axis of the goniometer and parallel to the specimen surface. Rotation about this axis may be controlled, for example, via a rod within the body of the goniometer that is attached to the specimen holding stage.
The problem with controlling a conventional goniometer is that an operator does not find that it is very intuitive which knob or knobs to turn (and in which direction) to achieve a desired result. This results from the fact that the image orientation displayed on a screen and the specimen orientation normally have some rotational phase relationship between them, which is not known in advance and worse yet is not ordinarily constant when the specimen position is being altered.
In a TEM, any change in position of the specimen irradiation spot along the axis of the incident electron beam results in a rotation of the image due to the magnetic lenses being used and the helical path that they cause the electrons to take. Any change in tilt angle of the specimen ordinarily results in some incidental change in position of the irradiation spot along the beam axis (due to the fact that ordinarily the axes of tilt rotation do not pass through the electron beam axis) and therefore causes some rotation of the image. Furthermore, whenever the specimen is tilted, any movement of the specimen in any one of the translational directions will also cause some movement of the incident spot in the direction of the beam axis and result in some rotation of the observed image.
As a result, the image tends to rotate at the same time as it makes a linear movement or a tilt movement, which makes it difficult for operators to navigate around a specimen while looking at the image. If a spot is seen on the screen near an edge of the field of view and it is desired to center the spot, for example, operators often inadvertently move the desired spot out of the field of view and have difficulty finding it again.
Another problem with manually controlled goniometers is that the range of possible travel for the stage is typically great enough that a collision can occur between the goniometer (or specimen) and a polepiece of the electron microscope. If a collision does occur, which is likely with inexperienced operators, damage can occur to a polepiece which is very expensive to repair. The polepieces of modern electron microscopes furthermore are getting closer and closer to each other (and to the specimen between them) because closer polepieces produce more precise images.
In Japanese Patent Application No. 2-49304 (filed Mar. 2, 1990 and corresponding to Kokai 3-254055 dated Nov. 13, 1991), an electron microscope is described in which motion commands for the goniometer are input to a processing system that calculates a limit for movability and transmits only motion commands within this limit to a drive system that implements them. It is said that this allows for setting the motion limit while correlation between the goniometer position and orientation are taken into consideration in order to allow movement only within a range such that the goniometer does not touch an objective stop. While this reference might have generally identified the objective of avoiding collisions, and might have generally suggested a way of preventing a collision if the operator inputs a motion command that would otherwise result in a collision, apparently no help is given to the operator in controlling sample motion (i.e., navigating around the sample) more intuitively and without getting lost.